I recently ported my landline (AT&T) to a new cellphone (Virgin Mobile). This took a week or 2. Then I ported my old cellphone number to GV. It cost $20. Finally, I forwarded my old cellphone number from GV to my new cellphone number (old landline). It works great!
You are correct that you can’t port your landline directly to GV. Transferring through a prepaid mobile number would be the best way to do that.
